1. Field of the Invention
The present invention relates to a stamper for replicating disks such as optical disks or VHD disks recorded with high-density data in an undulating pattern and a process for producing the same and, more particularly, to a more precise and denser patterning of the stamper.
2. Description of the Prior Art
A data recording disk such as an optical or VHD disk is recorded with data in an undulating pattern. In most cases, moreover, the disk is made of a resin and replicated by means of a stamper. This stamper and its producing process according to the prior art are described both on page 107 of pages 106 to 108 of "Production of Originals of Optical Disks by Lasers", National Technical Report, Vol. 29, No. 5 (October 1983), written by Yoshihiro Okino et al. and on pages 29 to 34 of "Development of Optical Video Disks for Business Use (or for image Location)", Hitachi Review, Vol. 65, No. 10 (1983), written by Koji Akatsu et al.
The precision of the depth of the undulating pattern affects the quality of a readout data signal and constitutes an important factor especially when the data form an image.
The prior art will be described with reference to FIG. 1.
A substrate 1 is coated with a photosensitive resist (i.e., photo resist) film 2 (as shown in FIG. 1(a)), which is then irradiated with an optical pattern 3 having data to be recorded (as shown in FIG. 1(b)) and is subsequently subjected to development. Then, the resist film is formed with such an undulating pattern as is shown in FIG. 1(c). This is called an original disk. This original disk is plated with nickel to produce a stamper 4 (as shown in FIG. 1(d)).
According to this process, the precision of the undulations of the stamper is dependent upon many factors including the thickness and uniformity of the photo resist film and the stability and reproductivity of the recording and recording steps. It is, however, very difficult and requires high technology and great skill to control those factors relating to the photo resist film. The yield of the finished products would be remarkably low even with this high technology and great skill.
The data disk may have a guide track as shallow as 0.07 .mu.m if it is of the write-once type (Reference should be made to page 26 of pages 23 to 34 of "High-Capacity Optical Disk File", Hitachi Review, Vol. 65, No. 10 (1983), written by Yoshito Tsunoda et al.). It is quite difficult to uniformly form that thin photo resist film. On a data disk in which the guide track and the address signal pit have different depths, moreover, it is necessary to give two kinds of depths to the undulating pattern to be formed in the photo resist film. These depths are controlled by the intensities of radioactive rays and by the developing conditions. However, this control is so difficult that the depths are irregular; consequently, the recording density and the S/N ratio of the readout data signal cannot be improved.
Since the precision of the depth deteriorates in the plating step, moreover, a novel invention is wanted for making the data disk highly dense from now on.